1. Field of the Invention
The present invention relates to a positive electrode current collector for a lead-acid battery having a coating on the surface.
2. Description of the Related Art
A lead-acid battery has a low energy density as compared with a nickel-metal hydride battery and a lithium ion battery. One of the causes is that lead or a lead alloy to be used as a positive electrode current collector is thick and heavy. Therefore, use of titanium or a titanium alloy (hereinafter, referred to as titanium or the like) for the positive electrode current collector and formation of a conductive oxide layer of tin dioxide or the like as a coating on the surface of titanium or the like have been proposed (reference to Japanese Patent Application Laid-Open (JP-A) No. 55-64377 and Japanese Patent No. 3482605). It is because use of titanium or the like makes a positive electrode current collector lightweight because of the lower specific gravity of titanium or the like than that of lead.
However, in the case where a positive electrode current collector made of solely titanium or the like is used for a lead-acid battery, although hard, titanium or the like is slightly dissolved in diluted sulfuric acid, which is an electrolytic solution, and accordingly there occurs a problem. Therefore, a tin dioxide which is insoluble in diluted sulfuric acid, which is an electrolytic solution, is formed as a coating on the surface of titanium or the like.
Moreover, in the case of forming tin dioxide on the surface of titanium or the like, there are advantageous points: (i) tin dioxide shows high electrochemical stability when it is positive electrode potential in an electrolytic solution; (ii) voltage decrease by existence of the coating of tin dioxide with low conductivity can be suppressed by titanium or the like with high conductivity; and (iii) since the melting point of titanium or the like is high, it can stand firing around 500° C. in a step of forming a tin dioxide coating.
As a method for forming the tin dioxide coating on the surface of a current collector substrate are generally employed a dip coating method and a spin coating method. Specifically, the coating of tin dioxide is formed by applying a raw material solution containing tin on titanium or the like and thereafter heating them. Further, a technique of forming a tin dioxide coating on the surface of a glass substrate by a spray method is also proposed (e.g., Japanese Patent No. 3271906, or J. J. Rowlette, American Chemical Society, 1052 (1986))
In Japanese Patent No. 3271906, in a first step, a first raw material solution obtained by dissolving an organic compound such as dibutyltin diacetate in an organic solvent is atomized in a spray form on the surface of a heated glass substrate. At that time, in order to avoid a factor of crystallinity decrease, an element such as antimony and fluorine having outermost electrons in a number higher by one than that of tin or oxygen is not added to the first raw material solution. The sprayed first raw material solution is thermally decomposed on the surface of the heated glass substrate to form an undercoating layer selectively oriented to a specified crystal plane. Next, a second raw solution obtained by adding an element having outermost electrons in a number higher by one than that of tin or oxygen to the first raw solution is atomized in a spray form on the surface of the undercoating layer.